Projection-type display devices and direct-viewing liquid crystal display devices are popular image display devices employing a display element that spatially modulates light emitted from a light source (lamp) for displaying video images by means of either transmissive or reflective optical modulation.
FIG. 20 shows a configuration of an example of a projection-type display device employing a transmissive liquid crystal panel.
Each component in FIG. 20 works as described below.
Lamp 105 illuminates liquid crystal panel 115 as a light source. Projection lens 116 magnifies and projects a video image displayed on liquid crystal panel 116 onto screen 117. Cooling fan 108 for controlling the lamp temperature cools down lamp 105 when it gets too hot.
Lamp driver 104 drives lamp 105. Fan control signal generator 107 drives cooling fan 108 for controlling the lamp temperature.
Liquid crystal panel driver 114 converts and processes video signals into signals required for driving liquid crystal panel 115, and displays images on liquid crystal panel 115.
The above components except for screen 117 are included in projector 118 (projection-type display device).
In a projection television (PTV), screen 117 is placed inside the display device, and images are projected from the back of the screen. In the case of a direct viewing liquid crystal display device, projection lens 116 and screen 117 are excluded from the above configuration, and the user looks at images directly on the liquid crystal panel.
Compared to an image display device employing a self-luminescent display element such as a CRT, black rise tends to occur more in dark portions in image display devices employing a display element for optical modulation such as liquid crystal panels.
One proposed method for suppressing this black rise and improving the contrast of displayed images is to dynamically change the lighting luminance of the lamp to match the input video images. For example, the Japanese Laid-open Application Nos. H5-127608 and H6-160811 disclose this method.
The Japanese Laid-open Application No. H5-127608 proposes the following method.
The characteristic of an input video signal is detected from the maximum and minimum values of the input video signal. When the mean of the maximum and minimum values is higher than a predetermined threshold, the luminance of the lamp is reduced to maintain the luminance of the displayed image within a certain range.
The Japanese Laid-open Application No. H6-160811 proposes the next method.
The maximum value of the input video signal is detected. If the maximum value is higher than a predetermined threshold, the luminance of the lamp is increased. If the maximum value is lower than the predetermined threshold, the lamp luminance is decreased. In addition, the amplitude of the video luminance signal when the maximum value is low is reduced compared to the amplitude of the video luminance signal when the maximum value is high. This enhances the relative contrast of images between the high and low maximum values.
As described above, when the luminance of the lamp, the light source, is dynamically controlled in accordance with the video signal, scenes with low or high lamp driving level are considered to continue for a long period in some input video sources.
If this state occurs with a discharge type light source such as high-pressure mercury lamp or xenon lamp, the lamp bulb temperature increases or decreases out of the reliability-assured range.
This causes so-called blackening or whitening by shading of the lamp bulb due to attachment of foreign particles or degradation of the lamp, resulting in reduced reliability of the lamp.